48 Notes oil Barometer Construction. 



a little below the mercurial surface ; the boiling out thus 

 proceeds from the closed end to within an inch of the open 

 end of the tube. The tube is now filled up with hot mercury, 

 and eventually it is suitably closed and inverted in its cistern 

 of boiled pure mercury. To what extent or how syphon 

 barometers are boiled out I am unable to state. Barometer 

 tubes may be boiled out or filled with warm mercury without 

 boiling out. The great standard barometer of Kew Obser- 

 vatory, which has a bore of one and one-tenth inches, 

 was filled by the aid of the air-pump, and without 

 boiling out. The Torricellian void above the mercurial 

 column is stated to have been, when the instrument was 

 completed, quite air free. I venture to express an opinion 

 that the boiling out of barometer tubes is a mistake. The 

 formation of oxide of mercury may not be grossly palpable ; 

 but I fear it is hardly possible to avoid the formation of some 

 oxide, and that the quantity, however small, may have its 

 effect upon the sensitiveness of the instrument. Possibly the 

 intervention of microscopic crystals of red oxide of mercury 

 between the metal and the glass may ultimately favour the 

 entrance of air into the void. The mode in which mercury 

 distils, and the absence of specific knowledge concerning any 

 power which mercury may possess of absorbing or occluding 

 gases, would appear to suggest that as far as the mercury 

 itself is concerned the boiling out is unnecessary ; or, if 

 necessary for depriving the mercury of air, or gas, or vapour, 

 of any kind occluded in its substance, as on that account 

 ineflfectual, for if the metal has this property it must soon 

 again take up what we have expelled at the exposed surface 

 in the cistern, and when saturated eliminate them into the 

 void, while all our experience of the comparative permanence 

 of the Torricellian vacuum renders this supposed property of 

 mercury improbable, the small and slow creeping in of air 

 being quite in unison with the fact of there being no real 

 adhesive contact between the metallic column and the glass 

 tube. Moreover, glass tubes, especially those of complex form, 

 are jeopardised by the boiling process. A carefully and fortu- 

 nately selected tube, well prepared, and therefore valuable 

 far beyond its money cost, may be broken during the boil- 

 ing by the turbulent and sudden bursts of mercurial 

 vapour; or, if not actually broken during the boiling, it 

 may be reduced to such a state of molecular unrest as 

 to break with apparent spontaneity, some time after it 



